1. Field of the Invention
This invention relates to a thermal printer for accurately printing bar codes and OCR (optical character recognition) characters as well as ordinary characters.
2. Prior Art
A conventional thermal printer is equipped with a font memory designed to store font patterns having a fixed number of vertical and horizontal dots representing, e.g., em or semi-em characters only. After the font pattern is read out of the memory, a between-line dot train is added between the font patterns so that the font pattern may be supplied to the printer as a dot pattern. Accordingly, the printer is used for printing ordinary characters only, and bar codes or irregular characters such as OCR characters which have widths different from that of ordinary characters cannot be printed. Consequently, the conventional thermal printer is incapable of printing a label including irregular characters in addition to ordinary ones.
In addition, bar codes pose special problems for thermal printers, for when a thermal printer is used to print a bar code, the contour line L as shown in FIG. 13a, of the bar code becomes extremely rugged, because each dot printed is close to the range of a circle and produces a decolorized portion B within a thick bar of the bar code. Thus, the accuracy of the bar printed by a thermal printer and detected by a bar code reader is substantially lower than the accuracy of a bar printed by a special bar code printer.